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Your grant says "incentive stock options." You're planning around the ISO tax treatment. But here's something most employees never learn: a chunk of those options might legally not be ISOs — and they'll be taxed in a completely different, more expensive way.
The culprit is a quiet provision called the $100K rule. If you have a large grant or fast vesting — common at startups and high-growth tech companies — this almost certainly affects you. Let me explain it so you don't get blindsided.
First, the stakes. Incentive stock options (ISOs) get favorable tax treatment: no ordinary income tax at exercise (though, as a previous post covered, they can trigger AMT), and the potential for long-term capital gains on the whole gain if you hold long enough.
Non-qualified stock options (NSOs) are taxed like a paycheck: when you exercise, the spread between your strike price and the market value is taxed as ordinary income right then — often with payroll taxes too. Same option mechanics, very different — and usually larger — tax bill.
So if some of your "ISOs" are quietly NSOs, your tax plan is built on a wrong assumption.
The rule is this: the value of ISOs that become exercisable for the first time in any one calendar year can't exceed $100,000 per employee. The value is measured using the stock's fair market value at grant (your strike price, essentially), times the number of shares vesting that year — not the current price.
Any options that vest in a year above that $100,000 threshold are automatically treated as NSOs for tax purposes, even though your paperwork calls them ISOs.
Say you're granted 40,000 options at a $5 strike, vesting 25% a year over four years — so 10,000 options vest each year. The grant-value test uses your $5 strike: 10,000 × $5 = $50,000 vesting per year. That's under $100K, so all of them keep ISO treatment. Clean.
Now change the strike to $15. Each year, 10,000 × $15 = $150,000 becomes exercisable. The first $100,000 of that stays ISO; the remaining $50,000 worth is reclassified as NSO. Same grant, same paperwork — but now part of every year's vesting is taxed as ordinary income at exercise instead of getting ISO treatment.
(Numbers are illustrative. Your real split depends on your strike, your vesting schedule, and any acceleration — confirm it with your equity docs and a CPA.)
Two reasons. First, the test uses the grant-date value, which feels disconnected from the much higher price the shares might be worth now — so people don't see it coming. Second, acceleration scrambles it. If a liquidity event, a cliff, or a change-of-control causes a big batch of options to become exercisable in the same year, you can blow far past $100,000 in that year and convert a large portion to NSO treatment unexpectedly.
It's not necessarily bad — NSOs aren't villains, and they don't carry the AMT complexity ISOs do. But it changes the math on when to exercise, how much tax you'll owe and when, and how to sequence everything. Planning around ISO rules for options that are actually NSOs is how people get surprised.
Pull your grant documents and your company's stock-plan portal. Many platforms (Carta, Shareworks, etc.) will actually flag the ISO vs. NSO split for you once you know to look. Map, year by year, how much of your vesting is ISO and how much has converted to NSO. Then build your exercise-and-sell plan on the real classifications — not the label on the grant. This is squarely a "model it before you act" situation, ideally with a CPA who handles equity comp.
The move: Not sure how much of your grant is actually ISO vs. NSO? Book a Clarity Call and we'll map your real split before you exercise.
Educational only — not personalized tax advice. Confirm your grant's classification with your equity docs and a qualified CPA.
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